The present invention relates generally to biomass conversion methods, and particularly to processes for treating lignocellulosic materials to increase their utility as carbohydrate sources.
Lignocellulosic materials in the form of wood and agricultural wastes represent an unexploited carbohydrate source for microbial fermentation, largely due to the resistance of lignocellulosic substrates to biodegradation. The various polymeric constituents of wood and other fibrous plant materials are intimately associated in a three-dimensional network whose susceptible bonds are largely inaccessible to physical, chemical, or enzymatic attack. In order to render these substances useful as substrates for microbial fermentation, one or more pretreatment processes must be employed to enhance the accessibility of the appropriate chemical bonds. Most known pretreatment strategies rely upon severe conditions, e.g., strong acid or alkali, to degrade lignocellulosic materials. Severe pretreatment processes typically involve large energy expenditures, reactant recycling or disposal problems, or significant capital investment. On the other hand, mild pretreatment systems provide only incremental improvements in the enzymatic or microbial digestibility of lignocellulosic materials. The following patents and publications disclose methods of treating cellulose-containing materials with ammonia or other amines:
Ulrey, U.S. Pat. No. 3,259,501, discloses a method of treating rice hulls involving contact with ammonia or other gaseous nitrogen compounds in a closed reactor, at temperatures from about 50.degree. C. to about 230.degree. C., and at pressures from about 70 kPa (10 psi) to about 6.21 MPa (900 psi). A treatment time of about 30 minutes is disclosed. The resulting treated rice hulls are claimed to be useful as livestock fodder or as a soil conditioner, mulch, or fertilizer.
Hultquist, U.S. Pat. No. 4,356,196, describes a process for treating alfalfa and other cellulosic agricultural crops. In this method, the materials to be treated are contacted with ammonia at pressures from about 203 kPa (30 psi) to about 4.05 MPa (588 psi), and at temperatures from about 10.degree. C. to about 85.degree. C., in a closed reactor, for about 30 minutes. Ammonia is then released from the reactor explosively, leaving a product having enhanced value as a foodstuff for livestock.
Dale, published European Patent Application No. 77,287, discloses a method for increasing the reactivity of cellulose, for example, cellulose-containing feeds such as alfalfa hay, involving contact with a cellulose-swelling agent, for example, gaseous or liquid ammonia, at a pressure from about 1.137 MPa (165 psi) to about 1.241 MPa (180 psi), and at a temperature of about 25.degree. C., for about 30 minutes. Pressure is then explosively released, providing a material with an expanded fiber structure.
Gallo, French Patent No. 2,518,573, discloses a process for saccharification of lignocellulosic materials involving pretreatment with an amine, for example, diethanolamine, for about 1 to 3 hours at a temperature from about 80.degree. C. to about 170.degree. C., followed by enzymatic hydrolysis.
A greatly improved process for pretreating wood and other lignocellulosic materials has now been developed, which relies upon use of ammonia in a supercritical or near-supercritical state to render the polymeric components of lignocellulosic materials susceptible to enzymatic hydrolysis. Ammonia exists as a supercritical fluid when maintained at a temperature greater than 132.4.degree. C., at a pressure greater than 11.28 MPa (1636 psi), and at a density greater than 0.235 g/mL. Lignocellulosic materials treated by the process of the present invention can be nearly completely hydrolyzed by cellulases, employed directly as carbohydrate sources for microbial fermentation, or fed to livestock.